Process for the manufacture of urea in granular form



April 19, 1960 A. GUYER ETAL 2,933,527

PROCESS FOR THE MANUFACTURE OF UREA IN GRANULAR FORM Filed Nov. 19, 1956 iilllllllllllililllillllillllll p mfumnz 22 24 a E lA NT Unit@ ,Staff-S Parenti@ PROCESS FR THE MANUFACIURE UREA v IN GRANULAR FORM August Guyer, Zurich, Fritz Marti, Basel, and V'Ivadeusz ,.jSztachelskL Visp, Switzerland, assignors to Lonza Electric and Chemical Works Ltd., Basel, Switzerland, a corporation of Switzerland Application November 19, 1956, serial No. 622,732

priority, application Switzerland April 13, 1956 s claims. (c1. 26o-55s) sures of'from about ZOO-350 atmospheres in an autoclave'V maintained at temperatures of 160-220 C. During the synthesis reaction the ammonia and carbon dioxide primarily combine exothermically to form ammonium carbamate which at the temperature of the reaction is transformed into urea and water. Theresulting reaction eluent contains in addition to urea and. water, uncombined residues of the starting materials and ammonium carbonate, bicarbonate and carbamate.

' It is also 4known to treat'v such. urea synthesis eiuent.

by a number of different procedures to recover from it the ammonia and carbon dioxide which have not been converted to urea and to vapori'ze the water so as to obtainl finally a relatively pure solid form of urea. For example, it has been proposed in U.S. Patent 2,267,133 to vaporize ammonia, carbon dioxide and a portion of the water from the eflluent while it is passing rapidly through an externally heated tube. The `mixture of liquid and gasY leaving the., tube is treated to separate` the gas from the liquid with the liquid thereafter being passed througha .packed tower in countercurrent flow toa stream of hot air. The degassed liquid (7S-85%V urea) may be obtained byjother known procedures and may be treated as vabovefor final drying or may be crystallized in acentrifuge. Also, ureasynthesis eluents havel been treated Vby'direct contact with steam for distillingthe solution up to 95-99%'fol1owed by prilling..V

The above'described processes for obtaining a pure solid form ofurea have many inherent disadvantages..

and shortcomings. Among the most detrimental are:

premature crystallization resulting in plugging of appara-A tus; increased quantities of decomposition products including biuret'in the solid urea because of long timeY temperature evaporation; highwater content in the solid urea requiring further drying with resultant product decomposition; losses of desiredproduct by decomposi tion;A and unusable product forms undersize granules.

such as oversize and In co-pending application Serial No. 622,731, there is disclosed a` method for overcoming the disadvantages ofV the above prior proposals and to provide a new andA improved method for the granulation of urea-containing solutions whereby the product urea granules are substantially water-free and have not had an appreciable increase in'percent of biuretl content. The invention disclosed in the above mentionedy application resides in the discovery ICCl ` that substantially water-hee urea granules of very low `completely undesirable because it causes turbidity of the solutions and, subsequently, destroys the brilliancy of biuret ycontent result from a very rapid partial evaporation of an aqueous 'urea solution under vacuum or at normal pressure followed by rapid cooling of the solution to just above the temperature of crystallization and substantially immediate granulation of the cooled solution in the `presence of solid urea ines. Through the use of suchgprocess it is possible to obtainV a free-owing granular product containing less than 2% of biuret, such product further having" anv extremely low water content, for example, 0.1 to v0.2% by weight, and a-crushing strength of about 3,00 grams per square millimeter. H owever, for many industrial and agricultural applications, rurea having a biuret content of 2% or more is unacceptable. F or instance, with formaldehyde in textile finishing and treating solutions, biuret contained in the urea is the textile linish. Further, in the plastics industry high biuret urea mustl be rejected as producing va turbid prod-1 fuct, While in the fertilizer and cattle feed industries high biuret urea has destroyed agricultural products and has 'been disclaim'ed as a safe fodder ingredient.

' Itis,4 an objectof the present invention to provide a new and-improved method of granulating aqueous solutions of urea'while at the some time decreasing the content of biuret or other decomposition products.

It is at furtherV object of the invention to provide animproved methodof granulating aqueous solutions ofl urea whereby the solid,l ureav product is substantially water-free and has a materially decreased biuret content.v

Other objects and advantages of the` invention will be apparent from theffollowing speciiication wherein preferred embodiments and details of the invention are described.

Accordingly, the Vpresent invention accomplishes the above objects bythe discovery that urea granules resulting from a granulation process including (l) rapid, high temperature, partial evaporation of an aqueous urea solution followed by (2) immediate cooling of such solution.

to just above the temperature of crystallization and (3) substantially immediate granulation in the presence of solid mrea rines, have a substantially reduced biuret content when the high temperature evaporation is carried out in the presence of, and the cooling is accomplishedV by ar gaseous phase of ammonia. Through the use of the above process, as described in further detail hereinafter, itis possible to obtain a free-liowing granular product containing less than 1% of biuret, such product further being substantially water-free and having a crushing strength of 300 grams per square millimeter or more;

The present invention not only provides a method of obtaining a substantially Aimproved solid Vurea product but also* has the additional advantages of simplicity of control, operation and equipment and reduced investment and operational costs.

'Ihe initial "short time partial evaporation of the aqueous urea solution may be accomplished in a filmtype evaporator wherein the solution is heated to from about l20-l80 C. for less than several minutes in the presence -of ammonia. nBy rapidly evaporating only vto the extent'whereby the solution concentration isV approximately 97-99% urea, there is substantially no increase in urea decomposition to form biuret and such biuret that is present or is formed is immediately ammoniated by the ammonia present whereby it may be partially"converted back to urea. VThe concentrated urea liquor leaving the evaporator is quickly cooled to a temperature which is just above the crystallization temperature, which may be about 13G-140 C., such rapid cooling also inhibiting the formation of biuret and further.

Patented Apr. 19, 1960 reducing the water content. x by countercurrent contact ofthe concentrated urea solution with gaseous ammonia in a packed tower or the like. @Cooling with ammonia'further reduces-urea conversion to biuret. The cooled/concentratedliquor peut passes directly to a granulation apparatus wherein 1t 1s I. Y immediately and intimatelyY mixed "with solid urea `ues (ratio ofjines to fresh meltfapproximately V1:1V to r3:1'l

The ureaVmelt-urea nespmixture quickly by weight). Y Y y crystallizes whereby granulation to a homogeneous` product is completed inY but a few feet of Vtravel through the i granulator with the resultant granules having a Vsolid e cooling is'accomplished concentrationY of 99.8% or-higherrand arfbiuretcontentY of less than 1% ofY the solidV urea. VTher granulation process Yincluding treatment of oversize and'undersize particles may be accomplished substantially vas disclosed in Serial No.622,73l. o o Y VThe inventionY isvfurther described by reference to the accompanying drawing in which the figure illustrates` a preferred embodiment of apparatus for accomplishing 'uV the purposes of the invention. o

YReferringpto thetigure, an aqueous urea solution toV be. granulated is introduced to an evaporator 1 through line 2. 'Such evaporator, which as shown is of agitated-lm design, consists` of a vertical heated cylinder 3 contain-v ing a rotor 4; The rotor is provided withrblades` 5l which approach the heated cylinder wall 3 with controlledv clearance. The agueous -urea solution as introduced through pipe- 2 Vis quickly formed into a thinV filmY by the rotating blades SV `and thel cylinder Wall 3. Thisjlm rapidly vspirals.downwardly andl is heated Vby indirect heat YeX- Y change through cylinder wall 3. The wall is heated;

by steam entering steam'jackets'r vand 7'lth'rough pipes: 8 and 9, respectively. After givingup heat to wall 3 the steanrin spaces and `7 is removed through pipes 10 and 11,-respectively. AmmoniaforV ammoniating the `Afilm of urea solution undergoingevaporation is intro-. ducedV throughl pipe 12. Such ammonia passes upwardly between theblades in contact with the aqueous lm and Y leaves the evaporator with freedwater vapor through pipe` evaporator 1 Vthrough pipe 14. At any given instant during operation there is little liquid solution contained in the evaporator andthe liquid contact time isY very short permitting highly Vsensitive evaporation.

The 97-99% urea melt leaving the evaporator. through Ypipe 14 substantially immediatelyfpasses through cooler 15, which may be a packed tower and through whichY relatively cool ammonia Vis passed, whereby the melt is cooled to a temperature a few degrees above its crystallization temperature. Ammonia for cooling enters through pipe 16 Vaudris removed with ammonia from pipe 12 by passage through the evaporator and pipe 13.

:Te ammonia` passing through the'cooler therebyaids ammonia front-pipe 12 in acting on biuret in the aqeous film within the evaporator. The cooled -urea ,melt leavesV the cooler through pipe 17 and valve 18' and is imme-.f-

Vdisintegrate and feed the material along wherebyV granulationoccurs iu but a few feet of materialtravel. A

t Vslight draftis maintained on they granulator with moist n airV beingV removed through pipe 23. Completely granulated urea is removed'through pipe 24-andis cooled and processed as described inv the aforementioned application. y t The ammonia used for'. cooling in4 cooler 15"mus't be'l substantially water-'free andprefera'bly at a' temperature u ofabout 2030 C. While the ammorgia;entering pipe 12 75 upwardly through the contact section ,26.* The `undis` solved ammonia remaining after' contact 'the scrubber is Ireturned to. evaporator 1 through line 12 and pump= 27. lSuch ammonia containsa smallunharnifuljauount of water vapor. The scrubbing medium for Vtrheloolunm enters as a spray from' lirieY 28 Vand consists of a por-A tion of the liquid bottoms 29, such bottoms being-comprised of water and dissolved 'ammonia'. The remaining portion of'bottoms 29 is rremoved'through line 30 to conventional distillation apparatus for separation of the ammonia and water. Water-freen ammonia from such distillationemay then be usedV4 as the coolant 16v forV cooler 1E.l A cooler 31 may be provided inline 29 whereby the scrubber`A V2S is maintained at a temperature ofaboutOCL'Y-j 1 When operating the above V`described d. apparatus or equivalent 'apparatus it has been observed that' anaqueous urea solution (7S-85% urea) may betevaporated in a tirst'stage, in the presence of'amrmonia, to a concentration of Y97-799176V urea Vin' les's than several/minutes at [temperatures of between 120fl80 Cfwitli substan-LA 1 dry urea solid lines in' a Ygranulat'or((ratio of nes toV tially no increase in biuret content.` Thenearly water- Y freie eiliuent isV ,their quicklycooled by `.ammonia tota teniperature'justY above its Y'temperature of crystallization' (130-140 C.) and is then rapidlymixed with cool melt, 1:1 to 3:1) wherebyV grauulationV immediately occ'urs as the meltis' cooled to Ybelow its ltnnperature*of crystallization, the product granules containing les'srthan 1% biu'retlV Y v While the manner of applioation of the inventionimay l beY variedV widely, ,particularly l*with regards ato'v specific apparatus designpthe following-example*,describes an operation of the process in apparatus designed` substantially as described and embodied in an actual urea granulation plant. In' the example parts areby weight.

1 Efxarrplef Y," An aqueoussolution Ycontaining 1000V parts urea aridv about 100 C. was' intronucedzrzoo kga/hf.) Yai anagrtated-lm evaporator heated by-sat'r'at'ed steam (pressure urea 'concentration at a temperature of 1605.Y The 978.5%,urea1melt wasv substantially immdiately'itrodced' to a cooler of packed tower'v design for" coultrcurrelitY contact with 40 parts of; dry ammonia having a temperature of about y25 Thecon'centratedurea melt leaving the cooler had an actual teinpefrafure''ofV 13'1'o C. (2C. above the temperature of crystallization) and was diately introduced to av pug mill: type granulatc` in adniiir-j ture with cool `dry'solid urea hes (ratio of ines to' melt,

by weight=2:l) having an'average temperature' o'f '50? C.Y Granulated urea and; fines dischargedY from thegranulator' Vat about 90"` C.we're further cooledrabout 60 C j Sani# ples of thergranules were tested' and found to'cont'ain'v less than 0.2% water. During evaporation the' urea solution was contacted within additiontotthe40 parts of anr-A monia from the,Y cooler, a vaporjcomprising540 parts" of ammonia and'f? parts water. The overhead vaporv from' the evaporator showed 245 parts water and` p'artsan'i monia. Prior to Vgranulation the"meltfwastestediascon-VV taining l2 parts water foi 1000 y parts'urea (appro,Xirn'zitely` 997% urea).

VSamples of"l the'l granules* produced werej tested and werev found to contain less than 028% biilretkr Whfas when operating thefprocess (evaporation andI cooling steps) without ammoniation and with the same proportions and conditions throughout, the granules showed a buiret content of about 2%.

The advantageous eects of short time, high temperature evaporation in the presence of ammonia followed by rapid cooling and immediate granulation of a degassed urea melt to obtain a substantially dry solid urea product containing less than 1% biuret will be apparent from the above example.

It will be appreciated that various modifications may be made in the invention described above without deviating from the scope thereof as defined in the appended claims.

We claim:

1. A process for the manufacture of substantially dry free-flowing granules of urea containing iess than 1% of biuret by Weight from an aqueous urea synthesis euent substantially free of unconverted ammonia and carbon dioxide: consisting essentially of passing said aqueous urea efuent through an evaporative heating zone; introducing gaseous ammonia into said heating zone; rapidly heating lsaid aqueous urea euent during its passage through said zone and in the presence of said gaseous ammonia at a temperature below about 180 C. and for a time suiiicient to form a concentrated urea eluent containing at least 97% urea by weight; immediately thereafter passing said concentrated urea effluent through a cooling zone wherein said eluent is cooled to a temperature within a few degrees above its crystallization temperature; immediately thereafter commixing said cooled concentrated urea eiiiuent with cool solid urea nes whereby said cooled eluent is rapidly cooled to below its crystallization temperature and additional water is evaporated by the heat of crystallization; and intimately mixing said mixture until granulation thereof to form the dry free-flowing solid urea granules containing less than 1% of biuret and less than 1% of water by weight.

2. A process for the manufacture of substantially dry free-flowing granules of urea containing less than 1% of biuret by weight from an aqueous urea synthesis effluent substantially free of unconverted ammonia and carbon dioxide: consisting essentially of passing said aqueous urea etiuent through an evaporative heating zone; intro ducing gaseous ammonia into said heating zone; rapidly heating said aqueous urea eiuent during its passage through said zone and in the presence of said gaseous ammonia at a temperature below about 180 C. and for a time suicient to form a concentrated urea euent containing at least 97% urea by weight; immediately thereafter passing said concentrated urea eiiiuent through a cooling zone wherein said effluent is cooled during contact with relatively cool gaseous ammonia to a temconcentrated urea euent with cool solid fines whereby said cooled effluent is rapidly cooled to below its crystal#V lization temperature and additional water is evaporated by the heat of crystallization; and intimately mixing said mixture until granulation thereof to form the dry freetlowing solid urea granules containing less than 1% of biuret and less than 1% of water by weight.

3. A process for the manufacture of substantially dry free-owng granules of lu rea containing less than 1% of biuret by weight from an aqueous urea synthesis effluent substantially free of unconverted ammonia and carbon dioxide: consisting essentially of passing said aqueous urea effluent through an evaporative heating zone; introducing a rst quantity of gaseous ammonia into said heating zone; rapidly heating said aqueous urea euent during its passage through said zone and in the presence of said gaseous ammonia at a temperature below about 180 C. and for a time suicient to form a concentrated urea euent containing at least 97% urea by weight; immediately thereafter passing said concentrated urea effluent through a cooling zone wherein said effluent is cooled during contact with a relatively cool second quantity gaseous ammonia to a temperature a few degrees above its crystallization temperature whereby the dissociation of urea to biuret is inhibited, said second quantity of ammonia thereafter passing through said perature within a few degrees above its crystallization temperature whereby the dissociation of urea to biuret is inhibited; immediately thereafter commixing said cooled heating zone as a portion of said first quantityv of ammonia; immediately thereafter commixing said cooled concentrated urea effluent with cool solid urea fines whereby said cooled eiuent is rapidly cooled to below its crystallization temperature and additional water is evaporated by thev heat of crystallization; and intimately mixing said mixture until granulation thereof to form the dry free-owing solid urea granules containing less than 1% of biuret and less than 1% of water by weight.

4. A process according to claim 3 wherein the ratio of cool solid urea lines to cooled concentrated urea eiuent during mixing thereof is from about 1:1 to 3 :1 by weight.

5. A process according to claim 3 wherein the short time evaporative heating of the aqueous urea synthesis eluent is carried out under vacuum.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Perrys Chemical Engineers Handbook, 3rd ed., Mc- Graw-Hill, New York, 1950, pp. 510; 1054-1055. 

1. A PROCESS FOR THE MANUFACTURE OF SUBSTANTIALLY DRY FREE-FLOWING GRANULES OF UREA CONTAINING LESS THAN 1% OF BIURET BY WEIGHT FROM AN AQUEOUS UREA SYNTHESIS EFFLUENT SUBSTANTIALLY FREE OF UNCONVERTED AMMONIA AND CARBON DIOXIDE: CONSISTING ESSENTIALLY OF PASSING SAID AQUEOUS UREA EFFLUENT THROUGH AN EVAPORATIVE HEATING ZONE, INTRODUCING GASEOUS AMMONIA INTO SAID HEATING ZONE, RAPIDLY HEATING SAID AQUEOUS UREA EFFLUENT DURING ITS PASSAGE THROUGH SAID ZONE AND IN THE PRESENCE OF SAID GASEOUS AMMONIA AT A TEMPERATURE BELOW ABOUT 180* C. AND FOR A TIME SUFFICIENT TO FORM A CONCENTRATED UREA EFFLUENT CONTAINING AT LEAST 97% UREA BY WEIGHT, IMMEDIATELY THEREAFTER PASSING SAID CONCENTRATED UREA EFFLUENT THROUGH A COOLING ZONE WHEREIN SAID EFFLUENT IS COOLED TO A TEMPERATURE WITHIN A FEW DEGREE ABOVE ITS CRYSTALLIZATION TEMPERATURE, IMMEDIATELY THEREAFTER COMMIXING SAID COOLED CONCENTRATED UREA EFFLUENT WITH COOL SOLID UREA FINES WHEREBY SAID COOLED EFFLUENT IS RAPIDLY COOLED TO BELOW ITS CRYSTALLIZATION TEMPERATURE AND ADDITIONAL WATER IS EVAPORATED BY THE HEAT OF CRYSTALLIZATION, AND INTIMATELY MIXING SAILD MIXTURE UNTIL GRANULATION THEREOF TO FORM THE DRY FREE-FLOWING SOLID UREA GRANULES CONTAINING LESS THAN 1% OF BIURET AND LESS THAN 1% OF WATER BY WEIGHT. 